This invention relates to an operating state control apparatus for an internal combustion engine, and more particularly, to an apparatus for controlling an operating state of an internal combustion engine, the operating state including at least output torque and intake air quantity of an internal combustion engine. These operating states are satisfactorily controlled on the basis of a dynamic model of the system relating to the operation of the internal combustion engine.
An internal combustion engine, as a prime mover, must stably realize a desired output in response to the manipulation of a driver. There is a tendency for the control of an internal combustion engine to be electronically performed so as to improve fuel consumption and realize a stable engine output.
Taking an air/fuel ratio control of an internal combustion engine in which a fuel injection amount is controlled as one example of such control, the control is performed, as in the fuel injection amount control apparatus whose structure is schematically shown in FIG. 2, according to classic feedback control theory. Namely, while the intake air quantity Q of an internal combustion engine E/G is determined by the opening degree of a throttle valve TH which opens and closes in response to an accelerator, a basic fuel injection amount Tp is obtained as Tp=K.times.Q/N (wherein K is a constant) on the basis of a load of the internal combustion engine E/G, this load being determined as Q/N from the above-mentioned intake air quantity Q and rotational speed N. Then, this basic fuel amount Tp is feedback controlled using a feedback correction factor F (A/F) and so on which is determined by a detection signal from means for detecting an air/fuel ratio of the intake air, such as an oxygen concentration sensor O.sub.2 provided to an exhaust system of the internal combustion engine E/G, and then a fuel injection amount .tau. for realizing target air/fuel ratio is obtained.
However, the apparatus for controlling the operating state of an internal combustion engine using such prior art techniques have suffered from the following problems.
(1) In normal internal combustion engines, the quantity of intake air is controlled by the opening degree of the throttle valve which is linked with the accelerator, and a fuel amount suitable for intake air quantity is mixed with the intake air by way of a carburettor or a fuel injection valve. Therefore, the output torque and fuel consumption amount are simply determined by only the operated stroke of the accelerator, and thus it has been impossible to precisely control fuel amount to a necessary output torque. In order to reduce fuel consumption amount, therefore, a way of control has been adopted so as to provide a lean air/fuel ratio in accordance with the operating state of the internal combustion engine.
However, when the air/fuel ratio is made large so that a lean air/fuel ratio mixture is combusted to improve fuel consumption of an internal combustion engine, there arises a problem that the output torque of the internal combustion engine drastically varies due to the variation of fuel supply amount caused from the air/fuel ratio control. FIG. 3 is a graph showing the relationship between air/fuel ratio A/F and output torque T of an internal combustion engine when comparing a large air/fuel ratio region with a small air/fuel ratio region, the variations .DELTA.Tr and .DELTA.Tl of the output torque T with respect to the variation in air/fuel ratio A/F can be found, as shown. The variation .DELTA.Tl in the large air/fuel ratio region is larger than the .DELTA.Tr in the small air/fuel ratio range. This means that engine operation when operating with a large air/fuel ratio (that is--a large numer), i.e. with a lean mixture, results in an unstable output torque. Namely, to stabilize the output torque during the operation in lean air/fuel ratio range has essentially been difficult using conventional feedback control in which a fuel supply amount is controlled in accordance with a detected concentration of oxygen in the exhaust system.
(2) As long as the control is performed such that fuel supply amount is determined on the basis of detected intake air quantity of an internal combustion engine, there necessarily occurs a time lag in the fuel supply amount control. Therefore, when intake air quantity is increased by steping onto the accelerator for acceleration, there arises a problem that output torque of the internal combustion engine first rises followed by the air/fuel ratio becoming lean, thereby generating a lean spike. This problem appears during deceleration to cause a rich spike to appear in the air/fuel ratio. In either case there arises a problem that a satisfactory acceleration/deceleration characteristic cannot be obtained because the reverse swing phenomenon occurs in the output torque characteristic required to the internal combustion engine.
Examples of such lean spike and rich spike are shown in FIG. 4.
(3) To solve the problem of the above-mentioned (2), an internal combustion engine control apparatus can be conceived (for example, "Accelerator Control Apparatus for Vehicles" disclosed in Patent Provisional Publication No. 59-122743) in which fuel supply amount is increased first when the accelerator is depressed, and then the intake air quantity is increased by opening the throttle valve with an arrangement that the throttle valve, which has conventionally been linked with the accelerator, is driven by way of an actuator. However, the control of the opening degree of the throttle valve encounters the following problems in connection with response and stability. Namely, in the conventional feedback control, in which controlled variables of an actuator is determined in accordance with the deviation of an actual opening degree from a target opening degree, if feedback gain is increased to increase the amount of feedback so as to provide a good driving feeling to the vehicle driver with the response of the control system being enhanced, excessive control would result and overshooting and/or downshooting would occur. On the other hand, if the amount of feedback is reduced to realize a stable control of intake air quantity, the follow-up characteristic is deteriorated and the driving feeling would be unsatisfactory. In this way, there is a contradiction in the conventional feedback control.
For this reason, therefore, the simple structure for controlling the throttle valve opening degree by way of an actuator or the like does not provide a perfect solution.
(4) On the other hand, as one method of controlling an internal combustion engine, an idea of controlling the internal combustion engine precisely using dynamic models of the internal combustion engine formed through so called modern control theory was proposed. This idea contemplates a providing stable control of output torque and air/fuel ratio with satisfactory response using parameters which are determined by dynamic models of the internal combustion engine using target output torque and target air/fuel ratio which are set from an amount of damands to the internal combustion engine. However, a response suitable for a given target value is just realized on the basis of dynamic models, and therefore, no control of minimizing fuel consumption is effected.
The present invention has been made so as to solve the problems in the above-mentioned (1) through (4), and contemplates providing an apparatus for controlling an operating state of an internal combustion engine with which apparatus engine output torque shows desired response and stability while fuel consumption amount can be made minimum.